1) How many deer would you expect in a natural system?In natural ecosystems the amount of browsing is dependent on the population level of the indigenous herbivores, which leads to the question: ‘What is the expected natural population level?’A standard model used in ecology is that of trophic levels, as first developed by Lindeman (1942). At the bottom level is the primary production, i.e. the biomass of plants produced through photosynthesis. Plants are eaten by the primary consumers (herbivores) which themselves are eaten by the secondary consumers (carnivores); in some food chains there can also be tertiary (and higher) consumers. As a rule of thumb, only 10% of the energy (biomass = chemical energy) passes up to the next level. Thus for, example, in a given area, every 100 kg of plant material produced annually will support 10 kg of herbivores and 1 kg of carnivores.Although a generalised and simplified model, this trophic level approach can be applied to the vegetation of northern Scotland as shown in Figure 2(a). The data come from various sources. The plant production figure comes from whole ecosystem production studies carried out at Moor House National Nature Reserve in the northern Pennines in the 1970s as part of the International Biological Programme (Heal et al. 1975). This indicated net annual plant production averaged over a range of habitats as 635 g dry wt m-2y-1, range 491-868. This equates to a photosynthetic efficiency of 1%, i.e. 1% of usable solar energy is converted to biomass. The study site is further south than Scotland, although at 550m altitude has a similar range of habitats. In Figure 2, a conservative value of 500 g dry wt m-2y-1 has been used.The dominant indigenous herbivore in the Highlands is the red deer (Cervus elephas scoticus). Armstrong (1996) gives the daily food intake of red deer and an average value of 1.75 kg dry matter per day has here been used. This figure gives a theoretical deer density of 78 km-2 assuming 10% of the plant production (biomass) passes to the primary consumers. This is a simplification in that it assumes that red deer are the only such consumer, whereas in practice there will be others dependent on location including roe deer (Capreolus capreolus), mountain hares (Lepus timidus), voles (Clethrionomys glareolus), red grouse (Lagopus lagopus scotica) and insects; in many areas, particularly in the locations of the north and west without domestic livestock, red deer are the only significant herbivore.However it does give an order of magnitude indication of the deer carrying capacity, which in practice will vary depending on the proportion and palatability of the actual vegetation types present. The same method indicates a carrying capacity of 131 blackface sheep, 23 Highland cows or 1,000 mountain hares based on the food intake data given in Armstrong (1996). The sheep figure equates well with St Kilda studies where the small Soay sheep in an unmanaged and unpredated environment have a varying density of 100-300 km-2 (Clutton-Brock and Pemberton 2004). Additionally, King and Nicholson (1964) show that in 1952 the densities of free-ranging sheep on upland farms in Scotland ranged from >25 km-2 in the far north to 167 km-2 in the south of the area, with the majority of the area having a density of 50-100 km-2.The wolf (Canis lupus) figures of 7 km-2 are added for interest, based on an average daily meat consumption of 5.53 kg per day (Glowaciński and Profus (1997), Stahler et al. (2006) indicate a range 2.77-10.4) and 65% water content of the meat.Figure 2(b) shows how the trophic levels would look, based on similar assumptions, when deer density is within the range 4-8 km-2, the level which is recommended for tree regeneration in the Scottish Highlands (Beaumont et al 1995, Staines 1995, Staines et al. 1995, Milne et al. 1998). It illustrates an unbalanced ecosystem with a herbivore density such that one square kilometre of land would not provide enough deer to support a single wolf.It also indicates that only 1% of the plant productivity is eaten, an order of magnitude less than would be expected from the theoretical model. This order of magnitude difference is one reason why it is so difficult to keep deer numbers low because there is enough food to support a much higher population; constant culling is needed to keep the population below the vegetation carrying capacity. It also indicates that the presence of wolves amongst unmanaged red deer would not bring the density down to 4-8 km-2.This trophic level model also assumes that herbivore population levels are largely determined by food supply as suggested by Milner et al. (2002): the greater the vegetation productivity, then the greater the expected animal population. What little research there has been on this topic shows that unmanaged populations of red deer in Scotland are somewhere between the theoretical density of 78 deer km-2 and the density of 4-8 km-2 necessary for woodland survival. Studies on the Letterewe Estate in the north west of Scotland and on the Isle of Rum indicate this density to be of the order of 16-18 red deer km-2 (Milner et al. 2002; Pemberton & Kruuk 2015). The Letterewe study also indicates that a population level of 16-18 red deer km-2 would result in 15% offtake of the vegetation biomass, a higher figure than the 10% of Figure 2, and yet resulting in a deer population level lower than the 78 km-2 calculated.This discrepancy between the theoretical carrying capacity and the actual population density can perhaps be explained by the seasonal nature of plant growth in the Highlands: most primary production takes place in the relatively short period of late spring and summer. Grazing at this time will be intense, with a high biomass offtake, as deer make up for their reserves lost over the winter and also have to support the current year’s calves. The summer plant production will support a large number of animals, more than can survive the lean winter months: as Milner et al. (2002) point out, it is the food availability during the unfavourable season which will be the ultimate determinant of herbivore populations and there is little palatable vegetation available during winter and early spring in the Highlands. This is reinforced by the conclusions of Pemberton and Kruuk (2015) who state that in the absence of culling or supplementary feeding, the population density of red deer on the island of Rum is strictly dictated by the overwinter carrying capacity of the land. Therefore it is not only the annual plant production which will determine the carrying capacity but also how much of this production the animals can store to maintain their metabolism through the unfavourable season.Additionally, the trophic level approach averages the vegetation production across the whole altitudinal range and landscape, and in practice significant tracts of land in the Highlands may have a lower productivity than that modelled, and hence would support a lower deer population than that modelled. There have been few studies of the overall vegetation productivity at the landscape scale across the Scottish Highlands and more research is needed on this topic. However the key point is that both the trophic level model and actual studies indicate that the vegetation is supporting a deer population significantly higher than that necessary to maintain woodland in the landscape.
ReferencesArmstrong H. 1996. The grazing behaviour of large herbivores in the uplands. Information and Advisory Note 47. Battleby: Scottish Natural Heritage.Beaumont D, Dugan D, Evans G, Taylor S. 1995. Deer management and tree regeneration in the RSPB reserve at Abernethy Forest. Scottish Forestry. 49, 155–161.Clutton-Brock TH, Pemberton J. 2004. Soay sheep: dynamics and selection in an island population. Cambridge: Cambridge University Press.Heal OW, Jones HE, Whitaker JB. 1975. Moor House UK. In: Rosswall T, Heal OW, editors. Structure and Function of Tundra Ecosystems. Stockholm: Ecological Bulletin 20, p. 295-320.Glowaciński Z, Profus P. 1997. Potential impact of wolves Canis lupus on prey populations in eastern Poland. Biological Conservation 80, 99-106.King J, Nicholson IA. 1964. Grasslands of the forest and sub-alpine zones. In: Burnett JH, editor. The Vegetation of Scotland. Edinburgh & London: Oliver and Boyd; p. 168-231.Lindeman RL. 1942. The trophic-dynamic aspect of ecology. Ecology 23:4, 399-417.Milner J, Alexander J, Griffin C. 2002. A Highland Deer Herd and its Habitat. London: Red Lion House.Milne JA, Birch JPD, Hester AJ, Armstrong HM, Robertson A. 1998. The impact of vertebrate herbivores on the natural heritage of the Scottish uplands – a review. Scottish Natural Heritage Review 95. Battleby: Scottish Natural Heritage.Pemberton JM, Kruuk LEB. 2015. Red deer research on the Isle of Rum NNR: management implications. Battleby: Scottish Natural Heritage.Staines BW. 1995. The Impact of red deer on the regeneration of native pinewood. In: Aldhous JR, editor. Our Pinewood Heritage. Edinburgh: Forestry Commission/Royal Society for the Protection of Birds/Scottish Natural Heritage; p.107–114.Staines BW, Balharry R, Welch, D. 1995. The impact of red deer and their management on the natural heritage of the uplands. In: Thompson, DBA, Hester AJ, Usher MB, editors. Heaths and Moorland: Cultural Landscapes. HMSO: London; p.294–308.Stahler DR, Smith DW, Guernsey DS. 2006. Foraging and feeding behaviour of the gray wolf (Canis lupus): Lessons from Yellowstone National Park, Wyoming, USA. Journal of Nutrition 136: 1923S-1926S.
WOODLAND OR OPEN GROUND? Scenarios for the persistence of woodland in the presence of grazing
Figure 2a. Trophic level diagram for the Highlands, showing expected numbers of deer and wolves assuming 10% biomass transferred upwards to the next trophic level; and assuming plant primary production of 500 tonnes km-1yr-1.
Figure 2b. Trophic level diagram for the Highlands assuming the same plant productivity and with deer density at 8km-2, the maximum possible for tree regeneration. This indicates that there would not be enough deer for a square kilometre of land to support even one wolf.